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  <div class="section" id="common-object-structures">
<span id="common-structs"></span><h1>Common Object Structures<a class="headerlink" href="#common-object-structures" title="Permalink to this headline">¶</a></h1>
<p>There are a large number of structures which are used in the definition of
object types for Python.  This section describes these structures and how they
are used.</p>
<p>All Python objects ultimately share a small number of fields at the beginning
of the object’s representation in memory.  These are represented by the
<a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject</span></code></a> and <a class="reference internal" href="#c.PyVarObject" title="PyVarObject"><code class="xref c c-type docutils literal"><span class="pre">PyVarObject</span></code></a> types, which are defined, in turn,
by the expansions of some macros also used, whether directly or indirectly, in
the definition of all other Python objects.</p>
<dl class="type">
<dt id="c.PyObject">
<code class="descname">PyObject</code><a class="headerlink" href="#c.PyObject" title="Permalink to this definition">¶</a></dt>
<dd><p>All object types are extensions of this type.  This is a type which
contains the information Python needs to treat a pointer to an object as an
object.  In a normal “release” build, it contains only the object’s
reference count and a pointer to the corresponding type object.
Nothing is actually declared to be a <a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject</span></code></a>, but every pointer
to a Python object can be cast to a <a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject*</span></code></a>.  Access to the
members must be done by using the macros <a class="reference internal" href="#c.Py_REFCNT" title="Py_REFCNT"><code class="xref c c-macro docutils literal"><span class="pre">Py_REFCNT</span></code></a> and
<a class="reference internal" href="#c.Py_TYPE" title="Py_TYPE"><code class="xref c c-macro docutils literal"><span class="pre">Py_TYPE</span></code></a>.</p>
</dd></dl>

<dl class="type">
<dt id="c.PyVarObject">
<code class="descname">PyVarObject</code><a class="headerlink" href="#c.PyVarObject" title="Permalink to this definition">¶</a></dt>
<dd><p>This is an extension of <a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject</span></code></a> that adds the <code class="xref py py-attr docutils literal"><span class="pre">ob_size</span></code>
field.  This is only used for objects that have some notion of <em>length</em>.
This type does not often appear in the Python/C API.
Access to the members must be done by using the macros
<a class="reference internal" href="#c.Py_REFCNT" title="Py_REFCNT"><code class="xref c c-macro docutils literal"><span class="pre">Py_REFCNT</span></code></a>, <a class="reference internal" href="#c.Py_TYPE" title="Py_TYPE"><code class="xref c c-macro docutils literal"><span class="pre">Py_TYPE</span></code></a>, and <a class="reference internal" href="#c.Py_SIZE" title="Py_SIZE"><code class="xref c c-macro docutils literal"><span class="pre">Py_SIZE</span></code></a>.</p>
</dd></dl>

<dl class="macro">
<dt id="c.PyObject_HEAD">
<code class="descname">PyObject_HEAD</code><a class="headerlink" href="#c.PyObject_HEAD" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a macro used when declaring new types which represent objects
without a varying length.  The PyObject_HEAD macro expands to:</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="n">PyObject</span> <span class="n">ob_base</span><span class="p">;</span>
</pre></div>
</div>
<p>See documentation of <a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject</span></code></a> above.</p>
</dd></dl>

<dl class="macro">
<dt id="c.PyObject_VAR_HEAD">
<code class="descname">PyObject_VAR_HEAD</code><a class="headerlink" href="#c.PyObject_VAR_HEAD" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a macro used when declaring new types which represent objects
with a length that varies from instance to instance.
The PyObject_VAR_HEAD macro expands to:</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="n">PyVarObject</span> <span class="n">ob_base</span><span class="p">;</span>
</pre></div>
</div>
<p>See documentation of <a class="reference internal" href="#c.PyVarObject" title="PyVarObject"><code class="xref c c-type docutils literal"><span class="pre">PyVarObject</span></code></a> above.</p>
</dd></dl>

<dl class="macro">
<dt id="c.Py_TYPE">
<code class="descname">Py_TYPE</code><span class="sig-paren">(</span>o<span class="sig-paren">)</span><a class="headerlink" href="#c.Py_TYPE" title="Permalink to this definition">¶</a></dt>
<dd><p>This macro is used to access the <code class="xref py py-attr docutils literal"><span class="pre">ob_type</span></code> member of a Python object.
It expands to:</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="p">(((</span><span class="n">PyObject</span><span class="o">*</span><span class="p">)(</span><span class="n">o</span><span class="p">))</span><span class="o">-&gt;</span><span class="n">ob_type</span><span class="p">)</span>
</pre></div>
</div>
</dd></dl>

<dl class="macro">
<dt id="c.Py_REFCNT">
<code class="descname">Py_REFCNT</code><span class="sig-paren">(</span>o<span class="sig-paren">)</span><a class="headerlink" href="#c.Py_REFCNT" title="Permalink to this definition">¶</a></dt>
<dd><p>This macro is used to access the <code class="xref py py-attr docutils literal"><span class="pre">ob_refcnt</span></code> member of a Python
object.
It expands to:</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="p">(((</span><span class="n">PyObject</span><span class="o">*</span><span class="p">)(</span><span class="n">o</span><span class="p">))</span><span class="o">-&gt;</span><span class="n">ob_refcnt</span><span class="p">)</span>
</pre></div>
</div>
</dd></dl>

<dl class="macro">
<dt id="c.Py_SIZE">
<code class="descname">Py_SIZE</code><span class="sig-paren">(</span>o<span class="sig-paren">)</span><a class="headerlink" href="#c.Py_SIZE" title="Permalink to this definition">¶</a></dt>
<dd><p>This macro is used to access the <code class="xref py py-attr docutils literal"><span class="pre">ob_size</span></code> member of a Python object.
It expands to:</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="p">(((</span><span class="n">PyVarObject</span><span class="o">*</span><span class="p">)(</span><span class="n">o</span><span class="p">))</span><span class="o">-&gt;</span><span class="n">ob_size</span><span class="p">)</span>
</pre></div>
</div>
</dd></dl>

<dl class="macro">
<dt id="c.PyObject_HEAD_INIT">
<code class="descname">PyObject_HEAD_INIT</code><span class="sig-paren">(</span>type<span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_HEAD_INIT" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a macro which expands to initialization values for a new
<a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject</span></code></a> type.  This macro expands to:</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="n">_PyObject_EXTRA_INIT</span>
<span class="mi">1</span><span class="p">,</span> <span class="n">type</span><span class="p">,</span>
</pre></div>
</div>
</dd></dl>

<dl class="macro">
<dt id="c.PyVarObject_HEAD_INIT">
<code class="descname">PyVarObject_HEAD_INIT</code><span class="sig-paren">(</span>type, size<span class="sig-paren">)</span><a class="headerlink" href="#c.PyVarObject_HEAD_INIT" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a macro which expands to initialization values for a new
<a class="reference internal" href="#c.PyVarObject" title="PyVarObject"><code class="xref c c-type docutils literal"><span class="pre">PyVarObject</span></code></a> type, including the <code class="xref py py-attr docutils literal"><span class="pre">ob_size</span></code> field.
This macro expands to:</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="n">_PyObject_EXTRA_INIT</span>
<span class="mi">1</span><span class="p">,</span> <span class="n">type</span><span class="p">,</span> <span class="n">size</span><span class="p">,</span>
</pre></div>
</div>
</dd></dl>

<dl class="type">
<dt id="c.PyCFunction">
<code class="descname">PyCFunction</code><a class="headerlink" href="#c.PyCFunction" title="Permalink to this definition">¶</a></dt>
<dd><p>Type of the functions used to implement most Python callables in C.
Functions of this type take two <a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject*</span></code></a> parameters and return
one such value.  If the return value is <em>NULL</em>, an exception shall have
been set.  If not <em>NULL</em>, the return value is interpreted as the return
value of the function as exposed in Python.  The function must return a new
reference.</p>
</dd></dl>

<dl class="type">
<dt id="c.PyCFunctionWithKeywords">
<code class="descname">PyCFunctionWithKeywords</code><a class="headerlink" href="#c.PyCFunctionWithKeywords" title="Permalink to this definition">¶</a></dt>
<dd><p>Type of the functions used to implement Python callables in C that take
keyword arguments: they take three <a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject*</span></code></a> parameters and return
one such value.  See <a class="reference internal" href="#c.PyCFunction" title="PyCFunction"><code class="xref c c-type docutils literal"><span class="pre">PyCFunction</span></code></a> above for the meaning of the return
value.</p>
</dd></dl>

<dl class="type">
<dt id="c.PyMethodDef">
<code class="descname">PyMethodDef</code><a class="headerlink" href="#c.PyMethodDef" title="Permalink to this definition">¶</a></dt>
<dd><p>Structure used to describe a method of an extension type.  This structure has
four fields:</p>
<table border="1" class="docutils">
<colgroup>
<col width="29%" />
<col width="21%" />
<col width="50%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Field</th>
<th class="head">C Type</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><code class="xref py py-attr docutils literal"><span class="pre">ml_name</span></code></td>
<td>char *</td>
<td>name of the method</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-attr docutils literal"><span class="pre">ml_meth</span></code></td>
<td>PyCFunction</td>
<td>pointer to the C
implementation</td>
</tr>
<tr class="row-even"><td><code class="xref py py-attr docutils literal"><span class="pre">ml_flags</span></code></td>
<td>int</td>
<td>flag bits indicating how the
call should be constructed</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-attr docutils literal"><span class="pre">ml_doc</span></code></td>
<td>char *</td>
<td>points to the contents of the
docstring</td>
</tr>
</tbody>
</table>
</dd></dl>

<p>The <code class="xref py py-attr docutils literal"><span class="pre">ml_meth</span></code> is a C function pointer.  The functions may be of different
types, but they always return <a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject*</span></code></a>.  If the function is not of
the <a class="reference internal" href="#c.PyCFunction" title="PyCFunction"><code class="xref c c-type docutils literal"><span class="pre">PyCFunction</span></code></a>, the compiler will require a cast in the method table.
Even though <a class="reference internal" href="#c.PyCFunction" title="PyCFunction"><code class="xref c c-type docutils literal"><span class="pre">PyCFunction</span></code></a> defines the first parameter as
<a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject*</span></code></a>, it is common that the method implementation uses the
specific C type of the <em>self</em> object.</p>
<p>The <code class="xref py py-attr docutils literal"><span class="pre">ml_flags</span></code> field is a bitfield which can include the following flags.
The individual flags indicate either a calling convention or a binding
convention.  Of the calling convention flags, only <a class="reference internal" href="#METH_VARARGS" title="METH_VARARGS"><code class="xref py py-const docutils literal"><span class="pre">METH_VARARGS</span></code></a> and
<a class="reference internal" href="#METH_KEYWORDS" title="METH_KEYWORDS"><code class="xref py py-const docutils literal"><span class="pre">METH_KEYWORDS</span></code></a> can be combined. Any of the calling convention flags
can be combined with a binding flag.</p>
<dl class="data">
<dt id="METH_VARARGS">
<code class="descname">METH_VARARGS</code><a class="headerlink" href="#METH_VARARGS" title="Permalink to this definition">¶</a></dt>
<dd><p>This is the typical calling convention, where the methods have the type
<a class="reference internal" href="#c.PyCFunction" title="PyCFunction"><code class="xref c c-type docutils literal"><span class="pre">PyCFunction</span></code></a>. The function expects two <a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject*</span></code></a> values.
The first one is the <em>self</em> object for methods; for module functions, it is
the module object.  The second parameter (often called <em>args</em>) is a tuple
object representing all arguments. This parameter is typically processed
using <a class="reference internal" href="arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal"><span class="pre">PyArg_ParseTuple()</span></code></a> or <a class="reference internal" href="arg.html#c.PyArg_UnpackTuple" title="PyArg_UnpackTuple"><code class="xref c c-func docutils literal"><span class="pre">PyArg_UnpackTuple()</span></code></a>.</p>
</dd></dl>

<dl class="data">
<dt id="METH_KEYWORDS">
<code class="descname">METH_KEYWORDS</code><a class="headerlink" href="#METH_KEYWORDS" title="Permalink to this definition">¶</a></dt>
<dd><p>Methods with these flags must be of type <a class="reference internal" href="#c.PyCFunctionWithKeywords" title="PyCFunctionWithKeywords"><code class="xref c c-type docutils literal"><span class="pre">PyCFunctionWithKeywords</span></code></a>.
The function expects three parameters: <em>self</em>, <em>args</em>, and a dictionary of
all the keyword arguments.  The flag must be combined with
<a class="reference internal" href="#METH_VARARGS" title="METH_VARARGS"><code class="xref py py-const docutils literal"><span class="pre">METH_VARARGS</span></code></a>, and the parameters are typically processed using
<a class="reference internal" href="arg.html#c.PyArg_ParseTupleAndKeywords" title="PyArg_ParseTupleAndKeywords"><code class="xref c c-func docutils literal"><span class="pre">PyArg_ParseTupleAndKeywords()</span></code></a>.</p>
</dd></dl>

<dl class="data">
<dt id="METH_NOARGS">
<code class="descname">METH_NOARGS</code><a class="headerlink" href="#METH_NOARGS" title="Permalink to this definition">¶</a></dt>
<dd><p>Methods without parameters don’t need to check whether arguments are given if
they are listed with the <a class="reference internal" href="#METH_NOARGS" title="METH_NOARGS"><code class="xref py py-const docutils literal"><span class="pre">METH_NOARGS</span></code></a> flag.  They need to be of type
<a class="reference internal" href="#c.PyCFunction" title="PyCFunction"><code class="xref c c-type docutils literal"><span class="pre">PyCFunction</span></code></a>.  The first parameter is typically named <em>self</em> and will
hold a reference to the module or object instance.  In all cases the second
parameter will be <em>NULL</em>.</p>
</dd></dl>

<dl class="data">
<dt id="METH_O">
<code class="descname">METH_O</code><a class="headerlink" href="#METH_O" title="Permalink to this definition">¶</a></dt>
<dd><p>Methods with a single object argument can be listed with the <a class="reference internal" href="#METH_O" title="METH_O"><code class="xref py py-const docutils literal"><span class="pre">METH_O</span></code></a>
flag, instead of invoking <a class="reference internal" href="arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal"><span class="pre">PyArg_ParseTuple()</span></code></a> with a <code class="docutils literal"><span class="pre">&quot;O&quot;</span></code> argument.
They have the type <a class="reference internal" href="#c.PyCFunction" title="PyCFunction"><code class="xref c c-type docutils literal"><span class="pre">PyCFunction</span></code></a>, with the <em>self</em> parameter, and a
<a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject*</span></code></a> parameter representing the single argument.</p>
</dd></dl>

<p>These two constants are not used to indicate the calling convention but the
binding when use with methods of classes.  These may not be used for functions
defined for modules.  At most one of these flags may be set for any given
method.</p>
<dl class="data">
<dt id="METH_CLASS">
<code class="descname">METH_CLASS</code><a class="headerlink" href="#METH_CLASS" title="Permalink to this definition">¶</a></dt>
<dd><p id="index-0">The method will be passed the type object as the first parameter rather
than an instance of the type.  This is used to create <em>class methods</em>,
similar to what is created when using the <a class="reference internal" href="../library/functions.html#classmethod" title="classmethod"><code class="xref py py-func docutils literal"><span class="pre">classmethod()</span></code></a> built-in
function.</p>
</dd></dl>

<dl class="data">
<dt id="METH_STATIC">
<code class="descname">METH_STATIC</code><a class="headerlink" href="#METH_STATIC" title="Permalink to this definition">¶</a></dt>
<dd><p id="index-1">The method will be passed <em>NULL</em> as the first parameter rather than an
instance of the type.  This is used to create <em>static methods</em>, similar to
what is created when using the <a class="reference internal" href="../library/functions.html#staticmethod" title="staticmethod"><code class="xref py py-func docutils literal"><span class="pre">staticmethod()</span></code></a> built-in function.</p>
</dd></dl>

<p>One other constant controls whether a method is loaded in place of another
definition with the same method name.</p>
<dl class="data">
<dt id="METH_COEXIST">
<code class="descname">METH_COEXIST</code><a class="headerlink" href="#METH_COEXIST" title="Permalink to this definition">¶</a></dt>
<dd><p>The method will be loaded in place of existing definitions.  Without
<em>METH_COEXIST</em>, the default is to skip repeated definitions.  Since slot
wrappers are loaded before the method table, the existence of a
<em>sq_contains</em> slot, for example, would generate a wrapped method named
<a class="reference internal" href="../reference/datamodel.html#object.__contains__" title="object.__contains__"><code class="xref py py-meth docutils literal"><span class="pre">__contains__()</span></code></a> and preclude the loading of a corresponding
PyCFunction with the same name.  With the flag defined, the PyCFunction
will be loaded in place of the wrapper object and will co-exist with the
slot.  This is helpful because calls to PyCFunctions are optimized more
than wrapper object calls.</p>
</dd></dl>

<dl class="type">
<dt id="c.PyMemberDef">
<code class="descname">PyMemberDef</code><a class="headerlink" href="#c.PyMemberDef" title="Permalink to this definition">¶</a></dt>
<dd><p>Structure which describes an attribute of a type which corresponds to a C
struct member.  Its fields are:</p>
<table border="1" class="docutils">
<colgroup>
<col width="29%" />
<col width="21%" />
<col width="50%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Field</th>
<th class="head">C Type</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><code class="xref py py-attr docutils literal"><span class="pre">name</span></code></td>
<td>char *</td>
<td>name of the member</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-attr docutils literal"><span class="pre">type</span></code></td>
<td>int</td>
<td>the type of the member in the
C struct</td>
</tr>
<tr class="row-even"><td><code class="xref py py-attr docutils literal"><span class="pre">offset</span></code></td>
<td>Py_ssize_t</td>
<td>the offset in bytes that the
member is located on the
type’s object struct</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-attr docutils literal"><span class="pre">flags</span></code></td>
<td>int</td>
<td>flag bits indicating if the
field should be read-only or
writable</td>
</tr>
<tr class="row-even"><td><code class="xref py py-attr docutils literal"><span class="pre">doc</span></code></td>
<td>char *</td>
<td>points to the contents of the
docstring</td>
</tr>
</tbody>
</table>
<p><code class="xref py py-attr docutils literal"><span class="pre">type</span></code> can be one of many <code class="docutils literal"><span class="pre">T_</span></code> macros corresponding to various C
types.  When the member is accessed in Python, it will be converted to the
equivalent Python type.</p>
<table border="1" class="docutils">
<colgroup>
<col width="45%" />
<col width="55%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Macro name</th>
<th class="head">C type</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>T_SHORT</td>
<td>short</td>
</tr>
<tr class="row-odd"><td>T_INT</td>
<td>int</td>
</tr>
<tr class="row-even"><td>T_LONG</td>
<td>long</td>
</tr>
<tr class="row-odd"><td>T_FLOAT</td>
<td>float</td>
</tr>
<tr class="row-even"><td>T_DOUBLE</td>
<td>double</td>
</tr>
<tr class="row-odd"><td>T_STRING</td>
<td>char *</td>
</tr>
<tr class="row-even"><td>T_OBJECT</td>
<td>PyObject *</td>
</tr>
<tr class="row-odd"><td>T_OBJECT_EX</td>
<td>PyObject *</td>
</tr>
<tr class="row-even"><td>T_CHAR</td>
<td>char</td>
</tr>
<tr class="row-odd"><td>T_BYTE</td>
<td>char</td>
</tr>
<tr class="row-even"><td>T_UBYTE</td>
<td>unsigned char</td>
</tr>
<tr class="row-odd"><td>T_UINT</td>
<td>unsigned int</td>
</tr>
<tr class="row-even"><td>T_USHORT</td>
<td>unsigned short</td>
</tr>
<tr class="row-odd"><td>T_ULONG</td>
<td>unsigned long</td>
</tr>
<tr class="row-even"><td>T_BOOL</td>
<td>char</td>
</tr>
<tr class="row-odd"><td>T_LONGLONG</td>
<td>long long</td>
</tr>
<tr class="row-even"><td>T_ULONGLONG</td>
<td>unsigned long long</td>
</tr>
<tr class="row-odd"><td>T_PYSSIZET</td>
<td>Py_ssize_t</td>
</tr>
</tbody>
</table>
<p><code class="xref c c-macro docutils literal"><span class="pre">T_OBJECT</span></code> and <code class="xref c c-macro docutils literal"><span class="pre">T_OBJECT_EX</span></code> differ in that
<code class="xref c c-macro docutils literal"><span class="pre">T_OBJECT</span></code> returns <code class="docutils literal"><span class="pre">None</span></code> if the member is <em>NULL</em> and
<code class="xref c c-macro docutils literal"><span class="pre">T_OBJECT_EX</span></code> raises an <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal"><span class="pre">AttributeError</span></code></a>.  Try to use
<code class="xref c c-macro docutils literal"><span class="pre">T_OBJECT_EX</span></code> over <code class="xref c c-macro docutils literal"><span class="pre">T_OBJECT</span></code> because <code class="xref c c-macro docutils literal"><span class="pre">T_OBJECT_EX</span></code>
handles use of the <a class="reference internal" href="../reference/simple_stmts.html#del"><code class="xref std std-keyword docutils literal"><span class="pre">del</span></code></a> statement on that attribute more correctly
than <code class="xref c c-macro docutils literal"><span class="pre">T_OBJECT</span></code>.</p>
<p><code class="xref py py-attr docutils literal"><span class="pre">flags</span></code> can be <code class="docutils literal"><span class="pre">0</span></code> for write and read access or <code class="xref c c-macro docutils literal"><span class="pre">READONLY</span></code> for
read-only access.  Using <code class="xref c c-macro docutils literal"><span class="pre">T_STRING</span></code> for <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-attr docutils literal"><span class="pre">type</span></code></a> implies
<code class="xref c c-macro docutils literal"><span class="pre">READONLY</span></code>.  Only <code class="xref c c-macro docutils literal"><span class="pre">T_OBJECT</span></code> and <code class="xref c c-macro docutils literal"><span class="pre">T_OBJECT_EX</span></code>
members can be deleted.  (They are set to <em>NULL</em>).</p>
</dd></dl>

<dl class="type">
<dt id="c.PyGetSetDef">
<code class="descname">PyGetSetDef</code><a class="headerlink" href="#c.PyGetSetDef" title="Permalink to this definition">¶</a></dt>
<dd><p>Structure to define property-like access for a type. See also description of
the <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_getset" title="PyTypeObject.tp_getset"><code class="xref c c-member docutils literal"><span class="pre">PyTypeObject.tp_getset</span></code></a> slot.</p>
<table border="1" class="docutils">
<colgroup>
<col width="20%" />
<col width="27%" />
<col width="53%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Field</th>
<th class="head">C Type</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>name</td>
<td>char *</td>
<td>attribute name</td>
</tr>
<tr class="row-odd"><td>get</td>
<td>getter</td>
<td>C Function to get the attribute</td>
</tr>
<tr class="row-even"><td>set</td>
<td>setter</td>
<td>optional C function to set or
delete the attribute, if omitted
the attribute is readonly</td>
</tr>
<tr class="row-odd"><td>doc</td>
<td>char *</td>
<td>optional docstring</td>
</tr>
<tr class="row-even"><td>closure</td>
<td>void *</td>
<td>optional function pointer,
providing additional data for
getter and setter</td>
</tr>
</tbody>
</table>
<p>The <code class="docutils literal"><span class="pre">get</span></code> function takes one <a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject*</span></code></a> parameter (the
instance) and a function pointer (the associated <code class="docutils literal"><span class="pre">closure</span></code>):</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="k">typedef</span> <span class="n">PyObject</span> <span class="o">*</span><span class="p">(</span><span class="o">*</span><span class="n">getter</span><span class="p">)(</span><span class="n">PyObject</span> <span class="o">*</span><span class="p">,</span> <span class="kt">void</span> <span class="o">*</span><span class="p">);</span>
</pre></div>
</div>
<p>It should return a new reference on success or <em>NULL</em> with a set exception
on failure.</p>
<p><code class="docutils literal"><span class="pre">set</span></code> functions take two <a class="reference internal" href="#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal"><span class="pre">PyObject*</span></code></a> parameters (the instance and
the value to be set) and a function pointer (the associated <code class="docutils literal"><span class="pre">closure</span></code>):</p>
<div class="highlight-c"><div class="highlight"><pre><span></span><span class="k">typedef</span> <span class="nf">int</span> <span class="p">(</span><span class="o">*</span><span class="n">setter</span><span class="p">)(</span><span class="n">PyObject</span> <span class="o">*</span><span class="p">,</span> <span class="n">PyObject</span> <span class="o">*</span><span class="p">,</span> <span class="kt">void</span> <span class="o">*</span><span class="p">);</span>
</pre></div>
</div>
<p>In case the attribute should be deleted the second parameter is <em>NULL</em>.
Should return <code class="docutils literal"><span class="pre">0</span></code> on success or <code class="docutils literal"><span class="pre">-1</span></code> with a set exception on failure.</p>
</dd></dl>

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